Antenna to antenna mutual coupling is typically undesirable. Where an omni-directional first antenna is located in close proximity to a second antenna different from the intended recipient antenna, the second antenna tends to absorb and re-radiate energy radiated by the first antenna when transmitting. Similarly, when the second antenna transmits some of its energy is absorbed by the first antenna, hence the coupling is mutual. Sometimes this is referred to as mutual impedance since one antenna acts as a high-Ohm resistor to the other's transmission. By the same principal when either antenna is receiving, energy that it could have captured is influenced by the other nearby antenna. Mutual antenna coupling affects both antenna transmit and receive performance, and the physical layout of antenna arrays are often designed with lambda half spacing, which avoids or minimizes this mutual coupling.
Current cellular and WiFi systems utilize MIMO antenna transmission techniques, and future cellular systems such as 3GPP 5G (LTE-A) contemplate massive MIMO transmissions from the network/eNB side. Massive in this regard refers to the total number of antenna elements in the array. These massive MIMO antenna arrays for 5 GHz LTE-A systems can easily become very large in physical size, mainly due to the relatively large wavelength λ which are in the range of 4 to 40 cm.
At the UE side a wavelength λ in the range of about 10 cm allows the handset designer to place only a very limited number of UE antennas. But beamforming gains at the UE side would be of great value; two such examples being powerful interference rejection combining (IRC) filters and down-selection of relevant multipath components (MPC) for the relevant channel components.
At the network/eNB side, massive MIMO arrays of size 16×32 elements with half-λ spaced antenna elements would conventionally exhibit a physical size of about 0.8 m*1.6 m. Implementing such a large physical size can be challenging for mobile operators who typically own the macro-cell sites in urban areas: larger antenna array sizes typically increases costs and accordingly the assumed performance; wind load becomes an increasingly important factor which may further increase implementation costs; and complaints about the aesthetics of and radiation from cell towers are likely to increase with increasingly large antenna arrays.
Embodiments of these teachings address some of the above issues by providing a physically smaller antenna array and increasing antenna performance in MIMO and beamforming scenarios.